1. Field of the Invention
The present invention relates to folded high frequency resonant cavity filters and, more particularly, to a high frequency resonant cavity filter that is folded by the use of a half-wave resonator such that capacitive cross-coupling between non-adjacent resonators within the filter is easily attained.
2. Description of the Prior Art
The use of a resonant cavity for high frequency filtering purposes is well known in the art. A resonant cavity housing generally contains a pair of coupling rods and a plurality of resonator rods. The shape of such a housing generally depends upon the number of resonator rods that are required within the housing to obtain a desired filtering characteristic. Also, a resonant cavity housing can be shaped to allow coupling between non-adjacent resonant rods, or capacitive cross-coupling.
Capacitive cross-coupling in resonant cavity filters is primarily used to cause an attenuation of poles at finite frequencies. This allows a decrease in the number of resonator rods that are required to meet a particular bandwidth specification, thereby reducing the required size of the resonant cavity filter housing. Also, since capacitive cross-coupling allows a decrease in the number of resonator rods, fewer finite Q elements are used, thereby decreasing insertion loss. Thus, capacitive cross-coupling in resonant cavity filters allows for a reduction in the size of the resonant cavity filter housing as well as a decrease in insertion loss.
To date, capacitive cross-coupling between non-adjacent resonator rods in resonant cavity filters has been mainly achieved through the use of coupling probes. These coupling probes are either mounted directly to the resonator rods or passed through walls within the resonant cavity housing that separate two or more resonator rods. A resonant cavity filter having coupling probes mounted directly to resonator rods is described in U.S. Pat. No. 4,216,448, entitled, Microwave Distributed-Constant Band-Pass Filter Comprising Projections Adjacent On Capacitively Coupled Resonator Rods to Open Ends Thereof, issued Aug. 5, 1980. A resonant cavity filter having coupling probes that pass through interior resonant cavity housing walls is described in German Patent No. DE3329057A1, issued Jan. 4, 1990. Transmission lines have also been used to couple between non-adjacent resonator rods.
Existing cross-coupled resonant cavity filters have many shortcomings. First, mounting coupling probes directly to resonator rods results in a degradation of resonator Q. Secondly, manufacturing resonant cavity filters having coupling probes tends to be complicated and costly due to a problem of securely attaching the coupling probes to the resonator rods and an increase in the number of assembled parts. Finally, since most existing folded resonant cavity filters are constructed such that all the resonator rods are positioned parallel to one another, the size of such filters occupy a substantial amount of vertical space. It is therefore desirable to overcome these shortcomings in constructing and using folded cross-coupled resonant cavity filters.